1. Field of the Invention
The present invention relates to a display device for performing display by controlling light-emitting operations of light-emitting elements, and particularly to a display device having light-emitting elements such as organic light-emitting diodes each having a light-reflecting surface on a back surface of a light-emitting layer.
2. Related Art
An organic light-emitting diode is a device for emitting light in such a manner that positive or negative electric charge is injected into a light-emitting layer of an organic thin film to thereby convert electric energy into light energy.
A display device constituted by organic light-emitting diodes (hereinafter referred to as “OLED display”) is of a self-luminous type to be different from a non-luminous type display device represented by a liquid-crystal display device. Hence, the OLED display is flat and lightweight because it is unnecessary to provide any auxiliary light source such as a backlight. In addition, the OLED display has a feature in that the OLED display is wide in viewing angle and rapid in response speed of display.
FIG. 19 is a partly schematic sectional view showing an example of the OLED display according to the related art. The OLED display is formed in such a structure that a transparent electrode 200 functioning as an anode, a hole transport layer 102, a light-emitting layer 100, an electron transport layer 101 and a, reflection electrode 300 of a light-reflecting metal functioning as a cathode are laminated successively on a transparent substrate 400.
When a DC voltage is applied between the transparent electrode 200 and the reflection electrode 300, holes injected from the transparent electrode 200 reach the light-emitting layer 100 via the hole transport layer 102 while electrons injected from the reflection electrode 300 reach the light-emitting layer 100 via the electron transport layer 101. The electrons are recombined with the holes to thereby emit light at a predetermined wavelength from the light-emitting layer 100.
Of the light emitted from the light-emitting layer 100, light traveling toward the transparent electrode 200 passes through the transparent electrode 200 and emerges from the transparent substrate 400. In addition, light traveling toward the reflection electrode 300 is reflected by the reflection electrode 300, passes through the light-emitting layer 100, the transparent electrode 200, etc. and also emerges from the transparent substrate 400.
Hence, to obtain a bright image in such an OLED display, it is important that a high reflectance electrode is used as the reflection electrode for increasing the amount of light emerging from the transparent electrode side.
Incidentally, in the structure, the reflection electrode serves as a high reflectance mirror when light has been not emitted yet. Hence, when the OLED display is used under a bright environment, there is a problem that image quality deteriorates because of reflection of a surrounding scene in the mirror, or there is a problem that the contrast ratio is lowered because of spoilage of black display by reflection of external light. As a measure to solve this problem, a structure in which a circularly polarizing plate 800 is disposed on the light-emergence surface side of the transparent substrate 400 has been put into practical use. The circularly polarizing plate 800 contains a polarizer 600, and a retarder 700 functioning as a quarter-wave plate.
The circularly polarizing plate 800 operates as follows. External light 2000 incident onto the OLED display from the environment is generally non-polarized light. When the external light 2000 passes through the polarizer 600, linearly polarized light having a plane of polarization in a specific direction is transmitted through the polarizer 600 but linearly polarized light having a plane of polarization perpendicular to the specific direction is absorbed to the polarizer 600. The linearly polarized light transmitted through the polarizer 600 is converted into circularly polarized light having a rotating plane of polarization (e.g., right-hand circularly polarized light) by the action of the retarder 700.
When the light passing through the retarder 700 is reflected by the reflection electrode 300, the phase of the light shifts by n to so that the light is converted into circularly polarized light rotating in the reverse direction (e.g., left-hand circularly polarized light). The light 2000R reflected by the reflection electrode 300 is made incident onto the retarder 700 again. When the light 2000R passes through the retarder 700, the light 2000R is converted into linearly polarized light by the action of the retarder 700. In this case, the linearly polarized light is absorbed to the polarizer 600, so that the light does not return back to the outside of the display device. That is, reflection of external light by the reflection electrode 300 is suppressed so that black display becomes darker. Hence, the contrast ratio is improved greatly.
Such a structure has been disclosed in Japanese Patent Publication No. JP-T-08-509834, Japanese Patent Publication No. JP-A-09-127885, etc.
Several methods for providing the OLED display as a full color display have been proposed and demonstrated. For example, there have been proposed a method using fluorescent color changing mediums (CCMs) in combination with blue light-emitting elements (hereinafter referred to as “CCM method”), a method using color filters of the three primary colors of red (R), green (G) and blue (B) in combination with white light emission (hereinafter referred to as “RGB-by-white method”), a method of directly applying pixels constituted by light-emitting elements of the three primary colors (R, G and B) in accordance with colors (hereinafter referred to as “RGB side-by-side arrangement method”), and so on.
In the CCM method, light emission of the three primary colors is obtained in such a manner that a fluorescence dye layer for changing fluorescent color is excited by light emitted from a blue light-emitting layer to thereby convert blue into green and red. In this method, device efficiency is lowered if color changing efficiency is low. In addition, under a bright environment, the color changing mediums are excited to be brighter by external light, so that black display is spoiled. Hence, the contrast ratio is lowered.
In the RGB-by-white method, light-emitting layers to be produced are of only a white type. This method has an advantage that the light-emitting layers can be produced most easily. There is however a problem that efficiency in utilization of light is lowered because light is absorbed to color filters.
In the RGB side-by-side arrangement method, it is necessary to produce three kinds of devices on one substrate. Hence, although the production process is somewhat complex, it is however said that this method is an ideal method in terms of efficiency because light loss is smallest. With respect to application (patterning) of RGB by colors, when a so-called low-molecular material such as a fluorescence dye or a metal complex low in molecular weight is used, a technique for applying RGB finely in accordance with colors by vacuum deposition of an organic layer using a shadow mask has been proposed.
As another method, a method using a polymer-based material such as a n-conjugated polymer or a dye-containing polymer containing a highly polymerized dye has been proposed. In this case, there has been proposed a technique in which RGB is provided so as to be finely patterned with an organic material in accordance with colors by printing in an ink-jet manner while banks of polyimide or the like are formed by photo-etching to thereby separate pixel regions from one another (The Institute of Image Information and Television Engineers Journal Vol. 54, No. 8, pp. 1115-1120).
Another example of the self-luminous type display device other than the OLED display is a plasma display. In the plasma display, each fluorescent substance is excited by ultraviolet rays generated by plasma discharge in a discharge cell to thereby obtain visible light with a predetermined color. That is, each fluorescent substance in the discharge cell functions as a light-emitting layer.
Generally, the fluorescent substance exhibits white under visible light and has characteristic of scatter reflection. Hence, external light incident onto the discharge cell is reflected by the fluorescent substance. Hence, under a bright environment, black display is spoiled by reflection of external light, so that the contrast ratio is lowered. As a measure to solve this problem, a plasma display panel having a reflective polarizer (reflective polarizing film) and an absorptive polarizer (absorptive polarizing film) at its front side has been disclosed in Japanese Patent Publication No. JP-A-2001-215886.
Since a half part of external light incident onto the plasma display panel from the environment is absorbed to the absorptive polarizer in this case, reflection of external light is reduced to be not larger than a halt. On the other hand, a half part of light emitted from the fluorescent substance is first transmitted through the reflective polarizer and the absorptive polarizer to contribute to display but the other half part of the light is reflected by the reflective polarizer. After the light reflected by the reflective polarizer is further reflected in the discharge cell, a half part of the reflected light is transmitted through the reflective polarizer and the absorptive polarizer to contribute to display but the other half part of the reflected light is reflected by the reflective polarizer again.
The aforementioned operation is repeated by many times. Finally, since almost all the light emitted from the fluorescent substance is not absorbed to but transmitted through the absorptive polarizer to contribute to-display, not only can lowering of luminance be suppressed by the disposition of the absorptive polarizer but also the contrast ratio under external light can be improved.
In the plasma display having a reflective polarizer and an absorptive polarizer at its front side, a half part of the external light incident onto the plasma display from the environment is absorbed to the absorptive polarizer but the other half part of the light transmitted through the absorptive polarizer is reflected in the discharge cell and then transmitted through the absorptive polarizer so that the resulting light goes out of the plasma display. Hence, reflection of external light cannot be suppressed perfectly.
On the other hand, a half part of light emitted from the fluorescent substance is first transmitted through the reflective polarizer and the absorptive polarizer to contribute to display but the other half part of the light is reflected by the reflective polarizer. The light reflected by the reflective polarizer is further reflected in the discharge cell and then goes toward the reflective polarizer again. On this occasion, the fluorescent substance which serves as a light-emitting layer generally does not retain but eliminates the state of polarization of light transmitted through the fluorescent substance or of light reflected. For this reason, the light reflected in the discharge cell becomes non-polarized light, so that a half part of the reflected light is transmitted through the reflective polarizer and the absorptive polarizer to contribute to display but the other half part of the reflected light is reflected by the reflective polarizer again.
In this manner, light emitted from the fluorescent substance contributes to display without being absorbed to the absorptive polarizer while reflection of the light is repeated between the reflective polarizer and the discharge cell.
Since reflectance of the discharge cell is not 100%, a part of the light is absorbed as a light loss whenever reflection of light is repeated between the reflective polarizer and the discharge cell. For this reason, all the light emitted from the fluorescent substance cannot contribute to display actually, and about 40% of light is cut off as a light loss.
On the other hand, in the OLED display having a circularly polarizing plate, reflection of external light by the reflection electrode can be reduced by the action of the circularly polarizing plate. Hence, a high contrast ratio can be achieved even under a bright environment.
Display however becomes darker because a part of light emitted from the light-emitting layer is absorbed to the circularly polarizing plate. This is because light emitted from the light-emitting layer is generally non-polarized light so that at least a half part of light is absorbed to a polarizer which is one of constituent members of-the circularly polarizing plate.
When a full color display device is achieved by organic light-emitting diodes, the RGB side-by-side arrangement method is used most preferably from the point of view of device efficiency. In the existing organic light-emitting diodes, however, it cannot be said that color purity is sufficient because the wavelength distributions of emitted light are broad and gentle in accordance with the colors.